Preheat the oven to 350°F Line muffin pans with paper cupcake liners.
Place the chopped chocolate in the top of a double boiler. (If you don't have a double boiler, use a wide stainless steel bowl that sits snugly over a saucepan one-third full of water.) The water beneath the bowl should be hot but not boiling. Stir the chocolate every minute or so until it's melted. As soon as the chocolate is melted, remove the pan from the heat and set aside to cool.
Sift the flour, baking powder, and salt onto a piece of waxed paper; set aside.
With a hand mixer or in the bowl of a stand-up mixer, thoroughly cream the softened butter and sugar until light and fluffy. Crack the eggs into a small bowl (just to monitor egg shells), and then add them to the mixture one at a time, beating well after each egg. Add the cooled chocolate and blend thoroughly. Add the sifted dry ingredients a little at a time so they don't fly up; beat at medium speed after each addition and after adding the last of the flour mixture, beat until smooth. Stir in nuts and vanilla extract.
Divide batter among prepared muffin cups and bake for about 15 to 17 minutes or until a toothpick inserted in the center comes out clean.
Transfer the cupcakes to a rack and allow them to cool completely before icing.